The present invention relates to a method of making a reservation with a subscriber who is engaged in telephone conversation in private branch exchange (PBX) or keyphone system, and more particularly a method that provides a periodic interruption to the subscriber and another subscriber who has a reservation with the highest priority and allows the subscriber to be linked with said another subscriber with the reservation without terminating the telephone conversation with the other subscriber who is currently engaged in the telephone conversation.
In general, when there are several subscribers who have made reservations for a telephone conversation with a specified subscriber, they are served in the order of their reservations according to a known reservation method of a telephone conversation. FIG. 1 is an example of such a reservation system, which is a switching system comprised of a main system 10 and subsets 40, 50. The main system consisting of a known configuration, accesses ROM 2 and RAM 3 in Central Processing Unit (CPU I), controls time switch (TSW) 13, and transmits and receives various data with the key subset 40 through transmission and reception (T/R) units 5, 6 as well as subset interface circuits 4a, 4b, and 4c. Another Central Processing Unit(CPU II) 20 provided in the subset 40, accesses ROM 21 and RAM 22, uses data transmission and reception (T/R) unit for data communication with the main system, processes control and display data from the main system 10, and sends key data entered from subset key matrix 24 to the main system 10 through the data transmission and reception unit 23. The Central Processing Unit 20 in the subset allocates combo-codec's channel by controlling voice control switch 27 and, when the handset 29 is on-hook, turns ON speaker 32 and microphone 33 and connects the speaker, the microphone and a tone ring to tone ringer 30.
FIG. 2 is a flow chart of a conventional method. Firstly, it will be described the conventional reservation method with reference to the circuit in FIG. 1 and the flow chart in FIG. 2. For the sake of convenience, the subsets 40, 50 and 60 in the FIG. 1 are each called first, second and third, subsets hereinafter. In the FIG. 1 if a first subscriber dials numbers for the second and third subsets through a keyboard of the key matrix and the LED matrix 24 when a second and a third subscribers are engaged in a telephone conversation through the second and third subsets, key entries are scanned periodically in the display and key scanning circuit 25 and then transmitted to the CPU II. When the CPU II sends this data to the CPU I through the data transmission/reception unit 23, the CPU I sends control signal to the CPU II asking it to send `busy` tones through the handset 29 of the subset 40, in step 2a, and have the CPU II control the voice control switch 27 to send `busy` tones through the handset 29 by the combo-codec 28. At that time, the CPU I sets transmission time length of the `busy` tone in the ROM 2. The CPU I checks the transmission time length of the `busy` tone in step 2b, and gives warning sounds through the speaker 32 in step 2j when the set time length is over, and cuts off the call. When the set time length is not over in said step, the CPU I checks if the handset of the subset 40 is on-hook in step 2c by checking the signals searched out from the CPU II.
When the handset 29 is on-hook in the step 2c, the CPU I checks, in the step cd, through CPU II, if a `hook flash` function was performed in the subset 40. If the hook flash was not performed in the step 2d, the CPU I returns to said step 2a, but if performed, it gives a confirming sound through the handset of the subset 40 in the step 2c and dials a call reservation code through the key and LED matrix 24. And then, the CPU I performs a scanning operation in the display and key scanning circuit 25 and makes entered data be processed in CPU II and it checks if the reservation buffer of the RAM 3 is full or not, in the step 2g. When the reservation buffer is full, the CPU I gives a warning sound through the handset of the subset 40 and cuts off the call. But if the reservation buffer is not full, in step 2h, the CPU I saves said reserved number entered in the reservation buffer of RAM 3 and gives a confirming sound through the handset 29 of the subset 40. Thereafter, the reservation function is completed by hanging up the handset.
Therefore, the conventional method had such inconvenience that the time length of the `busy` tone was always checked and, when it was over, the user had to hang up the handset. Thus, the user had to perform a hook flash sequence before the set time length of the `busy` tone was over, dial a designated call reservation code, hear its confirming sound and wait with the handset on-hook until the other party hung up his handset. That is, in the prior art method, when subscribers make reservation with another subscriber who is engaged in a telephone conversation, the reservations will be served in the sequence that they are made. Thus, even if a caller had to talk urgently to the subscriber engaged in the telephone conversation with the other subscriber, he had to wait for his turn, and the engaged subscriber didn't have any way to know whether any reservations were made with him.